1. Field of the Invention
The present invention relates to a charged-particle beam system equipped with metrology equipment using a laser interferometer.
2. Description of Related Art
A charged-particle beam lithographic system, such as an electron beam lithographic system, is available as a machine for creating semiconductor devices. Also, a charged-particle beam inspection system, such as an electron beam inspection system, is available as a machine for inspecting created semiconductor devices.
In such a system associated with fabrication of semiconductor devices, a workpiece from which semiconductor devices are lithographically fabricated or a workpiece on which semiconductor devices have been fabricated and which is to be inspected is placed on a workpiece stage. Under this condition, lithography or inspection is performed.
FIG. 1 schematically shows one example of an electron beam lithographic system. This system has an electron gun 1 emitting an electron beam. A condenser lens 2 focuses the beam onto a workpiece 4 on which a pattern is to be delineated, the workpiece 4 being placed on a workpiece stage 3. X- and Y-direction deflectors 5X and 5Y direct the beam at a position on the workpiece 4 according to positional data about a delineated pattern.
The workpiece stage 3 comprises an X-stage 3X for movement in the X-direction, a Y-stage 3Y for movement in the Y-direction, and a workpiece-holding stage 3H for holding the workpiece 4. The X- and Y-stages 3X and 3Y are driven by an X-stage drive mechanism 6X and a Y-stage drive mechanism 6Y, respectively.
Also shown in FIG. 1 are a reflective mirror 7, a laser head 8 for emitting laser light, an interferometer 9, and a metrology control circuit 10. A semitransparent mirror is positioned inside the interferometer 9 and splits the laser light from the laser head 8 into two beams traveling in different optical paths. One beam is made to impinge on the reflective mirror 7 mounted to the workpiece-holding stage 3H. The other beam is made to hit a reference reflective mirror disposed inside. The beams reflected from both reflective mirrors create interference fringes on the surface of the semitransparent mirror. The interferometer 9 produces a pulsed signal corresponding to each bright or dark portion of the interference fringes to the metrology control circuit 10 whenever the stage 3 moves a distance equal to the half wavelength.
A controller 11 sends a deflection signal corresponding to the positional data about the delineated pattern to the deflectors 5X and 5Y via D/A converters 12X, 12Y and via amplifiers 13X, 13Y. Also, the controller sends a stage-moving signal to the stage drive mechanisms 6X and 6Y via D/A converters 14X and 14Y. Furthermore, the controller sends a signal indicating a target position of the stage to the metrology control circuit 10.
A blanking mechanism 15 is composed of a blanking deflector 15D and a blanking plate 15P, and blanks the electron beam from the electron gun 1 in response to a blanking signal based on data about a pattern delineation time, the signal being sent in from the controller 11.
Also shown are D/A converters 16X, 16Y and amplifiers 17X, 17Y.
When a semiconductor pattern is delineated in practice, the electron beam from the electron gun 1 is focused onto the workpiece 4 by the action of the condenser lens 2. At the same time, the deflection signal based on the pattern data from the controller 11 is sent to the deflectors 5X and 5Y, so that the beam scans a desired area on the workpiece 4. Because of this scanning, a desired pattern is written on the workpiece.
In this processing for delineating a pattern, when the beam shifts from one field to the next on the workpiece, the controller 11 sends a stage movement signal to at least one of the stage drive mechanisms 6X and 6Y via the D/A converters 14X, 14Y to make a movement corresponding to one field. As a result, at least one of the X-stage 3X and Y-stage 3Y moves a distance corresponding to one field. The stage position taken after the movement is measured. The error between the measured position and the target position is corrected. The measurement and correction are described in further detail below.
In FIG. 1, the shown reflective mirror 7 and interferometer 9 are with respect to only one direction. In practice, an L-shaped reflective mirror 7L having two reflective side surfaces perpendicular to the directions of movements, or X- and Y-directions, are mounted on the workpiece-holding stage 3H as shown in FIG. 2. Laser light produced from the laser head 8 is split into two beams for measurements on the X- and Y-axes by a laser splitter 24. The direction of one beam is bent by a laser bender 25 for X-axis measurement, and the beam is made to impinge perpendicularly on one reflective surface of the L-shaped reflective mirror 7L via an X-direction interferometer 9X. The other beam is bent in direction by a laser bender 26 for Y-axis measurement and made to impinge perpendicularly on the other reflective surface of the L-shaped reflective mirror 7L via a Y-direction interferometer 9Y. Because of this structure, pulsed signals corresponding to amounts of movement of the X-stage 3X and Y-stage 3Y are sent to the metrology control circuit 10 by the interferometers 9X and 9Y, respectively.
Since signals (X0, Y0) indicative of the target position of the stage are sent to the metrology control circuit 10 from the controller 11, the circuit sends the errors (errors in stage stop position) between the signals from the interferometers 9X and 9Y and the target position signals (X0, Y0) to the amplifiers 13X and 13Y via the D/A converters 16X, 16Y and amplifiers 17X, 17Y. The amplifiers 13X and 13Y are connected to the deflectors 5X and 5Y. Accordingly, signals corresponding to the errors in the stage stop position are added to the deflection signals based on the pattern data. The resulting signals are supplied to the deflectors 5X and 5Y. Consequently, the errors in the stage stop position are corrected by deflection of the electron beam.
See, for example, U.S. Pat. No. 4,063,103.
When one field has been written and then the next field is to be written, the stage 3 is moved a distance corresponding to one field as described previously. Movement of the stage and deflection using the deflectors are made in the X- and Y-directions. For convenience of illustration, they will be described in relation to only one direction; description regarding the other direction is omitted.
Referring also to FIG. 3, it is now assumed that the stage is in a position Xa and that delineation of the field A is terminated at instant of time Ta. When the stage is in a position Xb, a field B will be written next.
When a movement signal corresponding to the position Xb is fed to the X-movement stage drive mechanism 6 from the controller 11, the stage 3 is started to be moved as shown in FIG. 3. The stage decelerates as it approaches the position Xb. At an instant of time Tb, the stage is almost at rest. In practice, however, the stage oscillates to and fro about the position Xb. This oscillation attenuates gradually. When the amplitude of the oscillation enters a tolerable range Vo at instant of time Tc, the field B is started to be delineated lithographically.
In delineating the field B, error (Vo/2) in the stage stop position is corrected by deflection of the electron beam. However, the following problem is produced at this time.
It takes some time (hereinafter referred to as the correction delay time) for the metrology control circuit 10 until a correction is made using the X-direction deflector 5X after the control circuit 10 measures the error (Vo/2) in the stage stop position based on the stage position signal from the interferometer 9 and on the stage target position signal from the controller 11. Therefore, movement of the stage made during the correction delay time is not corrected. Correspondingly, the electron beam is made to hit a position deviated from the desired position.
The correction delay time adversely affects the accuracy at which the pattern is lithographically delineated. Since semiconductor devices have been fabricated at quite high densities in recent years, the effect is quite great.
For example, the oscillation of the stage within the tolerable range Vo is regarded as a sinusoidal wave having a period of 100 Hz and an amplitude of ±1 μm. In this case, the correction delay time is 10 μsec. During this time interval, the stage moves about 6.28 nm. Accordingly, if a correction corresponding to error (Vo/2) in the stage stop position is made, positional error of about 6 nm is produced in the irradiation position. This error directly deteriorates the accuracy at which the pattern is lithographically written.